Step motors are rotary or linear motors using magnetic fields as the driving forces. FIG. 1 shows schematically a rotary step motor 10 comprising a stator 11 having four pairs of stator coils 12a, 12b; 13a, 13b; 14a, 14b and 15a, 15b, each pair of stator coils being aligned on opposite sides of the stator 11.
Rotatably mounted within the stator 11 is a rotor 18 which, for the sake of explanation, can be considered as a permanent magnet having a plurality of poles a, a'; b, b' and c, c'.
In operation, a pulse train is applied successively to each pair of stator coils 12a, 12b; 13a, 13b; 14a, 14b; and 15a, 15b so as, in effect, to generate a rotating magnetic field which induces similar rotation in the rotor 18 so that corresponding poles of the rotor 18 become aligned with the magnetic field produced by the energized stator coils.
Step motors are typically used in electromechanical systems wherein a rotary or mechanical movement is required whose magnitude is exactly proportional to a predetermined digital signal. The resolution of the digital component of such a system can easily be increased by increasing the number of bits of digital data from which the digital signal is composed. However, in order to translate the increased resolution of the digital signal into a corresponding increase in the resolution of the step motor, it is necessary to design the step motor so that each pulse applied to its stator coils results in a discrete but reduced rotation of the rotor. Thus, for example, if the resolution of the digital signal is increased by four, then each pulse applied to the stator coils of the step motor will reduce the angular rotation of the step motor by a factor of four.
In the simple arrangement shown in FIG. 1, each pulse applied to opposite coils of the stator 11 results in the rotor 18 rotating through an angle of 15.degree. . Thus, when the coils 14a and 14b are energized (no voltage being applied to any of the remaining stator coils), the rotor 18 will rotate through an angle of 15.degree. until the two poles b and b' of the rotor 18 nearest to the stator coils 14a and 14b are exactly aligned therewith. If now the stator coils 15a and 15b are energized (no voltage being applied to any of the remaining stator coils), then the rotor 18 will execute a further 15.degree. angular rotation until the poles a and a' become aligned with the opposite stator coils 15a and 15b. Since each pulse which is applied to respective stator coils results in a 15.degree. angular rotation of the rotor 18, the resolution of such a step motor 10 is only 15.degree..
It is known to increase the resolution of the step motor by energizing adjacent stator coils simultaneously by means of respective sinusoidal signals having identical amplitudes but being out of phase with respect to each other by 90.degree.. By this means, a driving signal applied to two adjacent stator coils results in a reduced rotation of the rotor 18 such that opposite poles thereof settle between the energized coils rather than in alignment with opposite stator coils.
The technique of energizing adjacent stator coils simultaneously in a step motor, is called micro-stepping and permits the effective resolution of the step motor to be increased many times such that high positioning tolerances can be achieved without loss in speed performance.
It has been shown that higher positional resolution of the step motor may be achieved when adjacent stator coils are energized with sampled sine and cosine phase currents (i.e. 90.degree. out of phase). FIG. 2 shows schematically a typical arrangement for driving a step motor by applying sine and cosine inputs 21 and 22, respectively, to adjacent stator coils thereof.
Each of the two input voltages 21 and 22 is fed to respective drivers 23 and 24, their respective d.c. offsets being adjustable by potentiometers 25 and 26 and their amplitudes being adjustable by means of potentiometers 27 and 28, respectively.
Typically, the drivers of the step motor 20 are calibrated by adjusting the d.c. offsets by means of the two potentiometers 25 and 26 (constituting offset adjustment means) until the resulting d.c. offsets of both the sine and cosine inputs are zero. The amplitudes of the two inputs are then adjusted by means of the potentiometers 27 and 28 (constituting amplitude adjustment means) until they are equal, the resulting symmetrical, equal amplitude input signals theoretically resulting in a balanced operation of the step motor 20.
Whilst such a method of calibration would, theoretically, result in perfectly balanced operation for an ideal step motor, in practice it is found that there exists a non-linear relationship between the angle of the sine and cosine inputs and the angular rotation of the step motor 20. This non-linear relationship is due to inaccurate electrical, magnetic and mechanical properties of the step motor 20. Thus, for example, the above-described method of calibration assumes that the adjacent stator coils are in all respects identical comprising identical numbers of turns and having precise relative orientation. This, of course, cannot be achieved in practice and results in a slight imbalance in the operation of the step motor 20 which is noticeable when the step motor 20 is used in applications requiring a very high resolution and smooth operation.